This invention relates to the conversion of coal and similar carbonaceous solids in the presence of an alkali metal-containing catalyst and is particularly concerned with the recovery of alkali metal constituents from spent solids produced during coal gasification and similar operations and their reuse as constituents of the alkali metal-containing catalyst.
It has long been recognized that certain alkali metal compounds can be employed to catalyze the gasification of carbonaceous materials such as coal and other carbonaceous solids. Studies have shown that potassium carbonate, sodium carbonate, cesium carbonate and lithium carbonate will substantially accelerate the rate at which steam, hydrogen, carbon dioxide, oxygen and the like react with bituminous coal, subbituminous coal, lignite, petroleum coke, organic waste materials and similar carbonaceous solids to form methane, carbon monoxide, hydrogen, carbon dioxide and other gaseous products. It has been found that of the alkali metal carbonates, cesium carbonate is the most effective gasification catalyst, followed by potassium carbonate, sodium carbonate, and lithium carbonate, in that order. Because of the relatively high cost of cesium carbonate and the low effectiveness of lithium carbonate, most of the experimental work in this area which has been carried out in the past has been directed towards the use of potassium and sodium carbonate. The catalytic activity of sodium carbonate, however, is substantially lower than that of potassium carbonate, therefore, attention has been focused in the past on the use of potassium carbonate as a gasification catalyst.
Coal gasification processes and similar operations carried out in the presence of alkali metal compounds at high temperatures generally result in the formation of chars and alkali metal residues. Coal and other carbonaceous solids used in such operations normally contain mineral constituents that are converted to ash during the gasification process. Although the composition of ash varies, the principal constituents, expressed as oxides, are generally silica, alumina, and ferric oxide. The alumina is usually present in the ash in the form of aluminosilicates. Studies indicate that at least a portion of the alkali metal compounds that are used as gasification catalyst constituents react with aluminosilicates and other ash constituents to form alkali metal residues containing water-soluble alkali metal carbonates, sulfates and the like and normally water-insoluble, catalytically inactive materials such as alkali metal aluminosilicates. Thus, the chars produced during coal gasification and similar conversion processes will contain, in addition to carbonaceous material and ash, alkali metal residues comprised of both water-soluble alkali metal constituents and water-insoluble alkali metal constituents. It is generally advisable to withdraw a portion of the char from the reaction zone during gasification and similar operations in order to eliminate the ash and alkali metal residues and prevent their building up within the reaction zone or other vessels in the system.
In gasification and other processes referred to above that utilize alkali metal-containing catalysts, the cost of the alkali metal constituents is a significant factor in determining the overall cost of the process. In order to maintain catalyst costs at a reasonable level, it is important that the alkali metal constituents be recovered and reused. One common method of recovering the alkali metal constituents is to wash the char particles removed from the reaction zone with water in order to leach out the water-soluble alkali metal constituents. Since the alkali metal is present in the form of both water-soluble and water-insoluble compounds, it has been found that only between about 70 and 80 percent of the alkali metal present in the char particles can normally be recovered by washing with water and therefore substantial quantities of makeup alkali metal compounds are required and add appreciably to the cost of the conversion process.
It has been proposed to recover the alkali metal constituents ties up as water-insoluble alkali compounds from the char particles along with the water-soluble alkali metal constituents by treating the char particles with lime in the presence of water at a temperature between about 250.degree. F. and 700.degree. F. The calcium ions from the lime evidently react with alkali metal aluminosilicates and other normally water-insoluble alkali metal compounds in the char particles to produce alkali metal constituents which dissolve in the water to form an aqueous solution. The resultant solution is recycled to the catalyst impregnation zone of the process where the alkali metal constituents are incorporated into the feed material for reuse as at least a portion of the alkali metal-containing catalyst. Although this procedure can result in the recovery of about 90 weight percent of the alkali metal constituents present in the char, the slurry effluent from the treatment step will contain a large quantity of fine lime particles which tend to make separations of the solids from the slurry effluent to produce the aqueous recycle solution difficult to achieve with a high degree of efficiency in a short period of time. More stages and larger equipment are normally required to effect such separations and this, in turn, results in an expensive extraction system which is costly to operate. Thus, there are major disadvantages connected with using the lime digestion technique to increase the recovery of catalyst constituents.